Hey man, sweet question.
I can't actually remember my first. I first played magic around
Onslaught. I went into my LGS to play some Pokemon TCG aged 12ish and saw some guys playing MTG. It was all dark and macabre with all the
onslaught zombies and my angsty little pubescent self who was looking for something darker loved it and I spent my pocket money on a couple of packs. Then mirrodin happened and all the folks at the LGS said it had broken everything and in my naivety I just believed what the bigger boys said and assumed the game was dead and not worth carrying on with. If only I knew then how much some magic players like to moan about MTG being dead. To this day I have no idea what I did with the cards from those first few packs.
Got back into it as an adult around Amonkhet block. Loved ancient Egypt as a kid and my buddy had just started playing it so I bought an Amonkhet bundle. First rare was
Glorybringer. Still love it to this day. Absolutely superb finisher in the aggro deck I ran back then. Still jam it in every deck I have with red now, even though I mostly play EDH and it's not as good there. Just love that dragon.
First mythic was from the same bundle.
Cruel Reality. Tried to make it work so many times. Really not that great to be honest. Also got a
Kefnet the Mindful in the same bundle, although technically I opened that second. Kefnet was "my card" in my playgroup for a long time. I love him. Such a beater.